The present invention relates to a method for controlling the operation of a high intensity discharge lamp and, more specifically for controlling a high intensity discharge lamp during lamp ignition and steady state operation.
A conventional high intensity discharge (HID) ballast, shown in FIG. 1, receives a power from an AC mains 100. A preconditioner/up-converter 110 includes circuitry for rectification of the AC power, power factor correction and for boosting of the AC peak voltage.
A regulated DC voltage (Vbus) is supplied to an inverter 120 (e.g. full-bridge type). The inverter output is supplied to the serial combination of a blocking capacitor Cs, inductor L and capacitor C. Inductor L and capacitor C form a resonant circuit. Inverter 120 typically operates at a switching frequency so as to be in an inductive mode (i.e. above the resonant frequency of the resonant circuit). Lamp 130 is connected across capacitor C.
A fixed voltage divider circuit 210 includes a plurality of resistors R1, R2, R3 and R4 and a pair of taps t1 and t1xe2x80x2 between resistors R1 and R2 and between resistors R3 and R4, respectively. A circuit 210 including a pair of voltage dividers divides down/sets the differential lamp voltage such that a sample 220 of the latter is at a conventional electronics level suitable for input to an integrated circuit 3. Integrated circuit 3 serves as a controller for controlling operation of lamp 130.
An ignitor (not shown) typically can produce ignition voltages of about 3.5 kilovolts (KV). In order to sample the lamp voltage during ignition, the lamp voltage is lowered or divided down to a usable level of less than 15 volts through circuit 210. Unfortunately, voltage divider circuit 210 will lower or divide down steady-state voltages, typically of about 200 volts, to less than 1-volt full scale. Voltages of such small magnitude are too small to yield good resolution, that is, are too small in magnitude to accurately reflect lamp operating conditions.
It is therefore desirable to provide a new and improved ballast which includes a sampling scheme for accurately reflecting lamp operating conditions during ignition and steady-state conditions.
In accordance with a first aspect of the invention, a ballast for powering a high intensity discharge lamp includes an inverter for supplying power to the lamp, at least one voltage divider for sampling a lamp operating condition and producing a sampled signal based on a scaling factor applied to the sampled lamp operating condition and a controller for driving the inverter in response to the sampled signal. The scaling factor varies based on the mode of lamp operation.
The sampled signal is always of a sufficient magnitude to yield good resolution. Consequently, lamp operating conditions can be accurately monitored. The magnitude of the sampled signal is maintained at an acceptable level by varying the scaling factor based on the mode of lamp operation.
It is a feature of this first aspect of the invention that the mode of lamp operation includes lamp ignition and steady-state lamp operation. Typically, the scaling factor applied is substantially larger during lamp ignition than during steady-state lamp operation. It is another feature of this first aspect that the at least one voltage divider includes two strings of resistors and further includes a pair of switches. Each switch selects which of the resistors from an associated string are to be coupled to the controller in response to instructions from the controller.
In accordance with a second aspect of the invention, a method of operating a high intensity discharge lamp ballast includes the steps of applying a scaling factor to a sampled lamp operating condition in producing a sampled signal, driving an inverter in response to the sampled signal, and varying the scaling factor based on the mode of lamp operation. The step of varying the scaling factor includes substantially reducing the scaling factor applied to the sampled lamp operating condition during steady-state lamp operation as compared to lamp ignition operation.
Accordingly, it is an object of the invention to provide an improved ballast which includes a sampling scheme for accurately reflecting lamp operating conditions during ignition and steady-state conditions.
Still other objects and advantages of the invention will, in part, be obvious and will, in part, be apparent from the specification.
The invention accordingly comprises several steps in the relation of one or more such steps with respect to each of the others, and a device embodying features of construction, combination of elements, and arrangements of parts which are adapted to effect such steps, all is exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.